In a trap, atoms can only exist in defined quantum energy levels. The Pauli Principle states that no two identical fermions can occupy the same quantum state at the same time. But bosons can. The Pauli Principle does not apply to bosons. At high temperatures, the atoms move freely between levels in the trap. But when a gas of bosons is cooled to an extremely low temperature, something very unusual happens. As the temperature gets colder, the motion of the atoms slows and they occupy lower and lower energy levels in the trap. Finally, when they reach a critical temperature, all of the atoms are in the lowest possible energy state. At this point, the bosons no longer behave as individuals but act as one and create a new state of matter—a Bose-Einstein Condensate or BEC.